The aluminum nitride has a high thermal conductivity and an excellent electrical insulating property and it has been used as a high thermal conductive substrate, a heat radiating component, and an insulating/heat radiating filler. Recently, semiconductor electronic components such as IC and CPU mounted on high-performance electronic devices, represented by a notebook computer, an information terminal and the like, are downsized and highly integrated. In line with that, downsizing has become essential for heat radiating members. Examples of the heat radiating member include a heat radiating sheet and a film-like spacer in which a high thermal conductive filler is filled in the matrix such as resin and rubber (Patent Document 1), a heat radiating grease in which a high thermal conductive filler is filled in silicone oil to have fluidity (Patent Document 2), and a heat radiating adhesive in which a high thermal conductive filler is filled in an epoxy resin (Patent Document 3). Further, examples of the high thermal conductive filler include aluminum nitride, boron nitride, alumina, magnesium oxide, silica, graphite, and various metal powders.
In order to improve the thermal conductivity of the heat radiating materials, it is important that a filler having a high thermal conductivity is highly densely filled. For this purpose, it has been urged to provide an aluminum nitride powder of a spherical shape having a particle diameter of from about several microns to several tens of microns.
Usually, the aluminum nitride powder has been produced by a reduction-nitridation method which fires alumina and carbon in a nitrogen atmosphere, a direct nitridation method which reacts metal aluminum directly with nitrogen, and a gas-phase method which reacts alkylaluminum with ammonia, and thereafter heats them.
However, the aluminum nitride powders obtained by the reduction-nitridation method and by the gas-phase method have shapes close to a sphere but their particle diameters are about submicron size.
On the other hand, according to the direct nitridation method, the aluminum nitride powder is obtained through the pulverization and classification, thereby it enables relatively easily to control the particle size and also enables to obtain the aluminum nitride powder having a particle diameter of from about several microns to several tens of microns. However, the pulverization step is essential in this method, therefore the particles of the obtained aluminum nitride powder have an angular shape and that causes a decrease in fluidity, with the result that it is difficult to highly densely fill the aluminum nitride powder obtained by this method as a filler in the resin.
In view of the above, a variety of methods have been studied in order to obtain the aluminum nitride powder of a spherical shape having a desired average particle diameter.
For example, a Patent Document 4 discloses a method for obtaining an aluminum nitride powder of a rounded shape having an average particle diameter of not less than 3 μm by firing a mixture of an alumina powder and a carbon powder in an inert atmosphere to form an aluminum oxide allowing particles thereof to grow and then firing (nitriding) the particles thereof in a non-oxidizing atmosphere containing nitrogen. However, there is a problem that the aluminum nitride powder obtained by this method has an elliptic shape and exhibits low sphericalness.
Further, Patent Document 5 discloses a method for producing an aluminum nitride powder by using a mixed powder of an aluminum oxide powder, a carbon powder and a rare earth compound as a starting material. With this method, compared with the general reduction-nitridation method, it enables to obtain the aluminum nitride powder having a relatively large average particle diameter. However, the average particle diameter is approximately 3 μm, and it is difficult to obtain the aluminum nitride powder having an average particle diameter exceeding 5 μm.
Further, a Patent Document 6 discloses a method for producing an aluminum nitride powder by developing an amorphous aluminum nitride powder in a flux comprising compounds of alkaline earth metals, rare earth metals and the like so as to assume a spherical shape, and thereafter, dissolving the flux to isolate the aluminum nitride powder. With this method, it enables to obtain the aluminum nitride powder having an excellent fluidity and filling property. However, there is a problem that impurities such as oxygen and the like easily infiltrate into the aluminum nitride powder during the heat treatment process.